Until the 1920's, practically all the internal combustion engines used only compression rings. Engine rotations were low and the compression rings were sufficient to control the small amounts of oil sent by the connecting rods to the cylinder walls.
With the use of higher rotation rate engines and the introduction of the pressurized lubrication system, the use of a specific ring was required to control the large amounts of oil sent against the cylinder walls. At that time, a ring named oil flow controller was developed. Such ring is characterized by having, on the external face thereof, two contact surfaces with the cylinder wall, which are separated by a channel containing radial slots for oil flow drainage. To increase the tangential load and ring conformability, a spring housing has been added to the internal face. From that time on, there has not been a great evolution in the basic shape, except for the rings with the external contact face presenting a conical profile for better control of the lubricant oil consumption during the break-in period of the engine.
The oil flow control ring must present two main functional characteristics: scraping the oil from the cylinder wall towards the crankcase of the engine, and maintaining a sufficient amount of oil to the compression rings above them, so that the oil film is maintained between the rings and the cylinders all the time.
The efficiency with which an oil flow control ring performs such functions results from the following factors: capacity of rapidly seating on the cylinder; the geometry of the contact face; the pressure against the cylinder walls; the dynamics of oil removal and drainage; and the ability to rapidly conform to the variations of the cylinder profile.
Nevertheless, the known prior art technology has limitations as to the optimization of friction losses and oil consumption.
It has been found that nearly 25% of friction losses of an engine are due to the rings, and 70% of said losses come from the oil ring under operation, mainly due to its high tangential load. Thus, any study relating the friction reduction in a set of piston rings should be started by the oil flow control ring.
The tangential load is directly related to the formula of the specific pressure that the ring contact face exerts against the cylinder wall. This pressure is one of the main parameters of oil flow control and is expressed by:
                              P          0                =                              2            ·            Ft                                d1            ·            c                                              (        1        )                            P0=specific pressure        Ft=tangential load        d1=nominal diameter of the ring        c=dimension of the ring contact face with the cylinder        
Experiments and literature show that the lower the specific pressure the higher will be the oil consumption. Thus, in order to maintain the same oil control conditions and reduce the friction loss by reducing the tangential load, a dimension reduction in the ring contact face with the cylinder is required to maintain the same specific pressure.
For example, considering an oil flow control ring with a coil spring (in a Diesel engine) and with a 102.0 mm nominal diameter, there is the following variation of the specific pressure as a function of the usual manufacturing tolerances:
d1 = 102.0 mmFt = 54 + 40% Nh = c/2 = 0.40 ± 0.10 mmFt min. = 54 NFt max. = 76 mmh min. = 0.30 mmh max. = 0.50 mm
From formula (1) above there would be:                P0 min.=1.06 N/mm2         P0 max.=2.48 N/mm2 which would give a variation of about 134% from the minimum value. As a function of this considerable difference in the specific pressure, a series of tests in dynamometer were performed, using oil flow control rings under the minimum specific pressure conditions, as compared to rings with maximum specific pressure, in order to study the performance of the engine in these two extremes (see table 1).        
TABLE 1Diesel engine - 4 cyl., 60 kw - 3000 r.p.mP0 = 1.06 N/mm2P0 = 2.48 N/mm2Lubricant oil0.570.26consumptionFriction loss19.821.1Wilan's lines(kW)
These results showed a large variation on engine's performance, mainly relating to oil consumption, partially caused by the dimensional tolerance of the contact surface. It should be noted that, by increasing the specific pressure, the consumption was reduced to half, but generating the undesired higher friction effect.
In addition to specific pressure, ring conformability is one of the most significant characteristics for oil flow control.
Ring conformability is a parameter that indicates the better or worse capacity of the ring to adapt to the possible deformations or diametral variations of the cylinder, thus maintaining its scraping and sealing capacity. The ring conformability is represented by the conformability factor that is expressed by the following mathematical relation:
  k  =            Ft      ·                        (                      d1            -            r1                    )                2                    4      ·      E      ·      I                      k=conformability factor        Ft=tangential load        d1=nominal diameter of the ring        r1=ring radial wall thickness        E=modulus of elasticity        I=moment of inertia        
The example below illustrates the conformability calculation presented herein:
By using mean dimensional values of an Otto cycle ring with a 67.1 mm diameter:                Ft=36.0 N        d1=67.1 mm        r1=2.23 mm        E=160 GPa        I=0.34        
The higher the k factor, the higher will be the capacity of the ring to adapt to cylinder deformations and better will be the oil flow control. Since it is not desirable to increase the tangential load due to the increase of friction losses, only the modulus of elasticity and the geometrical shape of the ring remain available to improve the conformability.